fix(fuse): list again when a listing changed while it was published (B159)

A lookup in the root publishes the "Shared with me" entry only if the root
listing did not change while its plan was read, and answered EAGAIN when it
had. Right after a restart the root is listed again as the first lookups come
in, so a write could fail with "Resource temporarily unavailable".
ensure_children now takes the listing again, up to four times.

docs/CHANGELOG.md

@@ -46,6 +46,8 @@ by its local id, in the queue, the stored tree and pins alike.
   lost Drive's answer to reading back the file's last upload** (B157).
 - **A file renamed twice in a row, while the first name was still taken, keeps the second name**
   (B158).
+- **Opening a file right after the daemon restarts no longer fails with "Resource temporarily
+  unavailable"** (B159).
 - **A file renamed just as its upload finished keeps its new name.** The mount no longer showed
   the old name, and a second rename no longer failed with "No such file or directory" (B152).
 - **A change that waits for a name to be freed no longer waits seconds after it is.** A rename or